13
Cobalt [Co, 27]
13.1 INTRODUCTION
Cobalt (Co), a metal of the group 9 in the periodic table of elements, reveals both
chalcophillic and siderophillic tendencies. Its content on the Earth’s crust is within the
range of 9–12 mg/kg. The highest Co contents are in mafic rocks, up to 200 mg/kg,
whereas in acidic igneous rocks, its contents range from 1 to 15 mg/kg. It is also likely
to be concentrated in black shales. Its distribution in sedimentary rocks is as follows
(in mg/kg): argillaceous (shales), 14–20; sandstones, 0.3–10; and limestones, 0.1–3.0.
Coal contains Co within the range 5–40 mg/kg. In fly ash, it may be accumulated up
to about 30 mg/kg.
Cobalt occurs mainly at +2 oxidation state, but also at +3 and +4. Its geochemical behavior is similar to Fe and Mn. Due to its siderophillic character, it is likely to
form minerals with S and As, such as linnelite, Co 3 S 4 ; smaltite, Co(Ni)As 3 ; cobaltite,
CoAsS; arsenosulfide, CoAsS; and safflorite, (Co,Fe)As 2 . In hypergenic zones and in
soils, it may occur as erythrite, Co 2 (AsO 4 ) 2 ·8H 2 O. Oxides of Mn, both abiogenic and
biogenic origins, reveal a great capacity to absorb Co.
World mine Co production (without the United States) in 2010 was 88 Mt, of
which Congo, 45; Zambia, 11; and Russia, 6.1 (USGS 2011). However, China is considered to be the world’s leading producer of refined Co, and as an important supplier
of Co to the United States. Recovery of Co is mainly from Cu–Co and Ni–Co sulfide
concentrates. It is also obtained from laterite and arsenide ores. In the United States
up to 25% of Co is obtained from recycling. About 50% of Co is used in superalloys
(strong, resistant to heat and corrosion), mainly for aircraft, and up to 20% in various
other metallic applications. It is also used (~30%) in a variety of chemicals.
13.2 SOILS
The worldwide mean value of Co content in soils has been estimated at 10 mg/kg.
The Co content of soils is inherited mainly from parent materials. Higher levels of Co,
mean 12 mg/kg, is often noticed in heavy loamy soils (Table 13.1). In other soils, its
mean contents do not exceed 7 mg/kg. However, in some specific locations, especially
where parent materials are enriched in Co, some maximum, elevated Co contents are
noticed (in mg/kg) in cambisols, 60; calcisols, 70; and histosols, 50. Soil-forming processes, which differ in various climatic zones, have a great impact on Co distribution
in soil profiles. Higher Co contents of surface layer of soils are in arid and semiarid climatic zones, whereas very low Co contents are in light soils, in humid climatic zones.
Soils over serpentine rocks contain Co up to 520 mg/kg. Its elevated contents (116–122
mg/kg) are also observed in ferralsols of various countries (Kabata-Pendias 2011).
Soils in arid and semiarid regions (e.g., Egypt) contain Co within the range 17–27 mg/
kg (Nassem and Abdalla 2003). Cobalt contents of soils of South Africa range from
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